The present invention relates to an improvement in a power steering control apparatus, used for a power steering system, for controlling a hydraulic reaction mechanism in accordance with the traveling velocity, the steering angle, and the like of a vehicle so as to obtain a desired steering force.
In power steering systems for reducing the steering wheel operating force (steering force) of a vehicle, various steering force control apparatuses for performing steering force control in accordance with various traveling conditions such as the traveling velocity and the steering angle of the vehicle by using hydraulic reaction mechanisms have been proposed. That is, a steering force needs to be controlled such that a light steering operation is allowed during parking or low-speed travel, and a steering operation is performed with a sense of rigidity during high-speed travel so as to ensure stability in straight travel. In order to perform such steering force control, input and output shafts in a power steering system are relatively rotated or restricted by using a reaction piston for selectively restricting the rotation of the input and output shafts in accordance with the magnitude of a reaction oil pressure.
As the most popular conventional steering force control apparatus of this type used for a power steering system, an apparatus having an arrangement disclosed in, e.g., Japanese Utility Model Laid-Open No. 62-25265 is known. In this arrangement, a reaction oil pressure branched from a part of a main hydraulic path extending to a power cylinder through a flow switching valve is used, and this pressure is controlled by a reaction oil power control valve constituted by a spool valve or the like to be guided to a hydraulic reaction chamber for driving a reaction piston.
According to steering power control apparatus having such a conventional arrangement, a filter for filtering a pressurized oil is generally arranged with respect to a branch portion of a branch path extending from a main hydraulic path to a hydraulic reaction chamber so as to prevent dust and foreign substances from entering a hydraulic reaction system. In this case, problems have been posed in terms of the shape and installation structure of the filter. More specifically, as shown in FIG. 10, the above-described branch portion extending from the main hydraulic path to the hydraulic reaction chamber is constituted by a path hole 2 serving as the main hydraulic path mechanically perforated in a steering body 1 and a path hole 3 serving as the branch path which is formed on the hydraulic reaction side so as to communicate with the hole 2. A substantially cylindrical filter 4 is fitted in a portion on the bottom side of the main path side path hole 2. Referring to FIG. 10, reference symbols 4a and 4b denote upper and lower ring-like frame members coupled to each other through a plurality of coupling pieces 4c and constituting the filter 4; and 4d, a cylindrical mesh portion mounted on the coupling pieces 4c between the upper and lower frame members 4a and 4b. A pressurized oil on the main path 2 side is filtered by the mesh portion 4d and is guided to the branch path 3 side. Reference numeral 5 denotes an inlet port pipe, inserted from above the upper portion of the path hole 2, for connecting a hydraulic pipe from a pump P (not shown); and 6, a small-diameter outlet side path hole which is formed to be continuous with the bottom side of the path hole 2 constituting the main path and extends to a power cylinder PS through a flow switching valve (not shown). In addition, reference symbol 2a denotes a tapered stepped outlet portion constituting the bottom portion of the path hole 2 when the path hole 2 is formed by a mechanical process using a drill or the like.
According to such a conventional structure, since the filter 4 is arranged in a space having a limited axial length between the upper side of the tapered portion 2a on the path hole 2 bottom side and the inner end of the inlet port pipe 5, the axial length of the filter 4 itself is limited. Since a sufficient axial length cannot be ensured between the upper and lower ring-like frame members 4a and 4b, the filter area of the mesh portion 4d is decreased, and required filter performance cannot be ensured. For this reason, a maintenance operation such as replacement of a filter due to clogging or the like must be frequently performed. In addition, according to the filter 4 having such a small filter area, a space of a sufficient volume cannot be ensured between the mesh portion 4d and the hole wall of the path hole 2 to which the path hole 3 around the mesh portion 4d is open, and a pressure loss is increased at this portion. As a result, a pressure to be supplied to the hydraulic reaction system is reduced, and problems are posed when a desired hydraulic reaction control is to be performed. Especially, if such a pressure loss is increased, the internal pressure of the filter 4 is increased, thereby posing problems that the mesh portion 4d is deformed outwardly, and the volume of a portion defined by the outer surface of the mesh portion 4d is decreased.
As described above, in the conventional filter structure, the upper and lower ring-like frame members 4a and 4b must have diameters substantially equal to the inner diameter of the path hole 2, and an operation of sequentially urging them into the path hole 2 to be assembled is required. Since the assembly operation is performed by urging the lower ring-like frame member 4b in the path hole 2 from its upper end side, the filter 4 tends to be deformed in the axial direction.
Especially, in a control spool valve or the like used in the above-described hydraulic reaction system, since a path system or the like has a small diameter, clogging tends to occur. Therefore, entrance of dust, foreign substances, and the like whose sizes pose no problem in the main hydraulic path extending from a pump to a power cylinder must be reliably prevented. In addition, when a filter is to be arranged in the branch portion extending from the main path to the hydraulic reaction control system, limitations in axial length pose problems in terms of filter areas and assembly performance as described above. Therefore, demands have arisen for a countermeasure by which all the above-described problems can be solved.
For example, Japanese Patent Laid-Open No. 63-38467 discloses a reaction oil pressure control valve used for the above-described hydraulic reaction control system. In such a steering power control apparatus, a choke portion is arranged on a main hydraulic path on the downstream side of a branch portion extending to a hydraulic reaction system. Therefore, a constant preset pressure is applied to the hydraulic reaction system to provide some rigidity to a steering wheel during straight travel or the like. Such a choke portion is conventionally arranged on the above-described main path portion on the downstream side of the filter portion. In this case, however, since the axial length of the filter portion is undesirably limited, problems are posed in formation of a required choke portion. More specifically, if the required choke portion is arranged on the downstream side of the filter portion, the height of the choke portion is further limited with respect to the filter. If a sufficiently long filter length is ensured, a sufficient length of the choke portion cannot be ensured. Therefore, a large choking amount is required. As a result, the flow of an oil pressure on the main path is disturbed, and other problems may be posed. Demands have arisen for a countermeasure by which these problems can also be solved.